This disclosure relates to measuring excess charge carrier lifetime in semiconductor samples using noncontact techniques. The lifetime describes how long, after injection, the excess charge carriers remain free to move before they are annihilated by recombination. The recombination can proceed via recombination centers related to impurities and defects that reduce lifetime values. Lifetime measurements can provide very sensitive diagnostic methods for monitoring such contamination and defects that in silicon are often related to metal contaminants (Fe, Cu, Ni) and to micro-defects involving oxygen. Long carrier lifetimes are especially important for performance of semiconductor devices that utilize excess free carriers, such as detectors and CCD imagers or photovoltaic devices, such as solar cells. High carrier lifetime and low surface recombination are desired for achieving high solar cell efficiency. Therefore, in silicon photovoltaics the measurement of excess carrier lifetimes can be used for initial prescreening and rejection of wafers with short lifetime. Carrier lifetimes can also be used for monitoring solar cell fabrication steps and prediction of cell performance.
Lifetime measurements versus steady-state illumination intensity and corresponding excess carrier injection level have important applications in silicon photovoltaics. Such characteristics are used for determination of the emitter saturation current and the open circuit voltage which are critical parameters for engineering high efficiency solar cells.
Carrier decay methods carry the advantage of direct lifetime determination without the need of any material and wafer parameters. The most common of these methods is the microwave detected photoconductance decay technique (μPCD) that is recognized for high speed, whole wafer lifetime mapping capability.